LTC3406ES5-1.8

Part: LTC3406 — Linear Technology Corporation

Type: Synchronous Step-Down Regulator

Description: 2.5 V to 5.5 V input, 600 mA synchronous buck regulator operating at 1.5 MHz with integrated switches and up to 96% efficiency.

Operating Conditions:

• Supply voltage: 2.5 V to 5.5 V
• Operating temperature: -40 °C to 85 °C
• Output voltage: 0.6 V to VIN (adjustable), or fixed 1.5 V / 1.8 V
• Switching frequency: 1.5 MHz

Absolute Maximum Ratings:

• Max supply voltage: 6 V (-0.3 V to 6 V)
• Max continuous current: 800 mA (P-Channel Switch Source Current / N-Channel Switch Sink Current)
• Max junction/storage temperature: 150 °C (Storage: -65 °C to 150 °C; Junction: 125 °C)

Key Specs:

• Regulated Feedback Voltage (VFB): 0.6 V (typ), 0.5850 V to 0.6150 V (-40 °C to 85 °C)
• Reference Voltage Line Regulation: 0.04 %/V typ, 0.4 %/V max (VIN = 2.5 V to 5.5 V)
• Peak Inductor Current (IPK): 0.75 A min, 1 A typ, 1.25 A max (VIN = 3 V, Duty Cycle < 35%)
• Input DC Bias Current (Sleep Mode): 20 μA typ, 35 μA max (VFB = 0.62 V, ILOAD = 0 A)
• Input DC Bias Current (Shutdown): 0.1 μA typ, 1 μA max (VRUN = 0 V, VIN = 4.2 V)
• Oscillator Frequency (fOSC): 1.2 MHz min, 1.5 MHz typ, 1.8 MHz max (VFB = 0.6 V)
• P-Channel FET RDS(ON): 0.4 Ω typ, 0.5 Ω max (ISW = 100 mA)
• N-Channel FET RDS(ON): 0.35 Ω typ, 0.45 Ω max (ISW = -100 mA)

Features:

• High Efficiency: Up to 96%
• Low Quiescent Current: 20 μA during sleep operation
• 1.5 MHz Constant Frequency Operation
• No Schottky Diode Required
• 100% Duty Cycle for Low Dropout Operation
• Internal Synchronous Rectifier switches

Applications:

• Cellular Telephones
• Personal Information Appliances
• Wireless and DSL Modems
• Digital Still Cameras
• MP3 Players
• Portable Instruments

Package:

• 5-lead plastic TSOT-23 (ThinSOT)

The ● denotes specifications which apply over the full operating temperature range, otherwise specifications are T A = 25 ° C. VIN = 3.6V unless otherwise specified.

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In most applications the LTC3406 does not dissipate much heat due to its high efficiency. But, in applications where the LTC3406 is running at high ambient temperature with low supply voltage and high duty cycles, such as in dropout, the heat dissipated may exceed the maximum junction temperature of the part. If the junction temperature reaches approximately 150 ° C, both power switches will be turned off and the SW node will become high impedance.

To avoid the LTC3406 from exceeding the maximum junction temperature, the user will need to do some thermal analysis. The goal of the thermal analysis is to determine whether the power dissipated exceeds the maximum junction temperature of the part. The temperature rise is given by:

$$T_R = (P_D)(\theta_{JA})$$

where P D is the power dissipated by the regulator and θ JA is the thermal resistance from the junction of the die to the ambient temperature.

The junction temperature, T J , is given by:

$$T_J = T_A + T_R$$

where T A is the ambient temperature.

As an example, consider the LTC3406 in dropout at an input voltage of 2.7V, a load current of 600mA and an ambient temperature of 70 ° C. From the typical performance graph of switch resistance, the R DS(ON) of the P-channel switch at 70 ° C is approximately 0.52 Ω . Therefore, power dissipated by the part is:

$$P_D = {I_{LOAD}}^2 \bullet R_{DS(ON)} = 187.2\text{mW}$$

For the SOT-23 package, the θ JA is 250 ° C/W. Thus, the junction temperature of the regulator is:

$$T_J = 70^\circ\mathrm{C} + (0.1872)(250) = 116.8^\circ\mathrm{C}$$

which is below the maximum junction temperature of 125 ° C.

Note that at higher supply voltages, the junction temperature is lower due to reduced switch resistance (R DS(ON) ).